How do I get a sofa through an extremely small front door ?

As the title suggests, I have a very small front door.

I'm finding it difficult to buy a sofa because I just don't know how to get it through.

I should add i'm clueless about measurements (maths was never my strong point) and so I was wondering if someone could give me some advice for DUMMIES i.e. very child-like advice to help me understand.

Maybe I will then be able to get a sofa through the door.

Perhaps some advice on how to pop a window would help too.

Thank you.